Evaluation of a modified gen-probe amplified direct test for detection of Mycobacterium tuberculosis complex organisms in cerebrospinal fluid.

Laboratory evidence for tuberculous meningitis is difficult to acquire due to the low numbers of organisms present in cerebrospinal fluid (CSF) and the presence of nucleic acid amplification inhibitors. The Amplified Mycobacterium tuberculosis Direct Test (MTD) is sensitive and specific for the direct detection of M. tuberculosis complex in respiratory samples but has not been approved for CSF. We evaluated a modified version of the current MTD, optimized for use with CSF samples. Samples were prepared by spiking CSF with various numbers of M. tuberculosis complex organisms. The modified MTD performance was compared with results obtained using a purified RNA sample extracted using the Qiagen RNeasy Protect Bacteria Mini Kit. By use of CSF artificially spiked with M. tuberculosis complex, the sensitivity of the modified MTD was 100% (six of six) for CSF samples containing approximately 600 CFU/ml, 78% (seven of nine) for approximately 60 CFU/ml, 50% (three of six) for 6 CFU/ml, and 17% (one of six) for samples with <1 CFU/ml. The specificity of the modified MTD method was 100% (22 of 22). The sensitivity of the Qiagen MTD method was 100% for CSF samples containing approximately 600 CFU/ml (six of six) and approximately 60 CFU/ml (nine of nine), 50% for samples with approximately 6 CFU/ml (three of six), and 50% for samples with <1 CFU/ml (three of six). The specificity of the Qiagen MTD method was 86% (19 of 22). With the Qiagen MTD method, however, initial results were equivocal for 14 of the 27 (52%) positive samples, requiring repeat analysis, whereas with the modified MTD, only 1 of 27 (4%) was equivocal. The modified MTD for CSF samples was less time-consuming and less expensive and resulted in considerably fewer equivocal results than the Qiagen MTD method did.